LT1120AC データシートの表示（PDF） - Linear Technology

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LT1120AC

Micropower Regulator with Comparator and Shutdown

Linear Technology 

LT1120A

APPLICATIONS INFORMATION

Reference

Internal to the LT1120A is a 2.5V trimmed class B output

reference. The reference was designed to be able to source

or sink current so it could be used in supply splitting

applications as well as a general purpose reference for

external circuitry. The design of the reference allows it to

source and sink typically 4mA. The available source and

sink current decreases as temperature increases. It is

sometimes desirable to decrease the AC output imped-

ance by placing an output capacitor on Pin 6. The reference

in the LT1120A becomes unstable with large capacitive

loads placed directly on it. When using an output capaci-

tor, about 20Ω should be used to isolate the capacitor

from the reference pin. This 20Ω resistor can be placed

directly in series with the capacitor or alternatively the

reference line can have 20Ω placed in series with it and

then a capacitor to ground. This is shown in Figure 1. Other

than placing large capacitive loads on the reference, no

other precautions are necessary and the reference is

stable with nominal stray capacitances.

REF

6

OUTPUT

20Ω

OR

10µF

REF

6

20Ω

OUTPUT

10µF

1120A AI01

Figure 1. Bypassing Reference

Overload Protection

The main regulator in the LT1120A is current limited at

approximately 250mA. The current limit is stable with both

input voltage and temperature.

Like most other IC regulators, a minimum load is required

on the output of the LT1120A to maintain regulation. For

most standard regulators this is normally specified at

5mA. Of course, for a micropower regulator this would be

a tremendously large current. The output current must be

large enough to absorb all the leakage current of the pass

transistor at the maximum operating temperature. It also

affects the transient response; low output currents have

long recovery times from load transients. At high operat-

ing temperatures the minimum load current increases and

having too low of a load current may cause the output to

go unregulated. Devices are tested for minimum load

current at high temperature. The output voltage setting

resistors to the feedback terminal can usually be used to

provide the minimum load current.

Frequency Compensation

The LT1120A is frequency compensated by a dominant

pole on the output. An output capacitor of 10µF is usually

large enough to provide good stability. Increasing the

output capacitor above 10µF further improves stability.

In order to ensure stability, a feedback capacitor is needed

between the output pin and the feedback pin. This is

because stray capacitance can form another pole with the

large value of feedback resistors used with the LT1120A.

Also, a feedback capacitor minimizes noise pickup and

improves ripple rejection.

With the large dynamic operating range of the output

current, 10000:1, frequency response changes widely.

Low AC impedance capacitors are needed to insure

stability. While solid tantalum are best, aluminum

electrolytics can be used but larger capacitor values may

be needed.

1120afa

6

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